The classical Abbe diffraction limit in fluorescence light microscopy had been a barrier for obtaining image information with a resolution better than half the wavelength of light for quite a long time. As the frontiers of science and technology approach the nanoscale, defeating the diffraction limit is a prerequisite to get nanometer resolution, especially in for biological samples. Several recent methods are capable of resolving structure beyond the diffraction limit, but these sub-diffraction techniques suffer from shortcomings. For example, photoactivated localization microscopy and stochastic optical reconstruction microscopy methods of localizing individual fluorophores in the sample to sub-diffraction precision require the generation of a large amount of raw images. Also, stimulated-emission depletion requires a sub-diffraction illumination of light and spot-by-spot scanning of the sample, which leads to low speed and requires a strong driving laser field.
Another high-resolution method, structured illumination microscopy (SIM), has been of special interest in recent years. Linear SIM was previously realized with resolution limitation, but more recently nonlinear SIM has become a widely used method to get a high-resolution image. In nonlinear SIM, two counter-propagating fields construct a spatially periodic illumination structure. The atoms or molecules have different fluorescence abilities under different light intensities. This dependence is nonlinear rather than the atom or molecule undergoing the multiphoton's process; in practice, the nonlinear order is less than four. This generates high spatial frequency information in the far field, hence leading to higher resolution. However, high resolution means higher-order nonlinearity and subsequently high light intensity. This leads to the possibility for damage to the sample and thus limits the use of this kind of microscopy, especially in biological systems. Wei et al. (Nano Lett., 10, 2531, 2010) use metal plasmons to construct a periodical strip pattern in a microscopy method called plasmon structure-illumination microscopy. However, due to the limitations of metal plasmons, this method has poor resolution and low precision.